CASCADING BCD COUNTERS BASIC INFORMATION

BCD counters are used when the application
involves the counting of pulses and the result of counting is to be displayed
in decimal. A single-stage BCD counter counts from 0000 (decimal equivalent
‘0’) to 1001 (decimal equivalent ‘9’) and thus is capable of counting up to a
maximum of nine pulses.

The output in a BCD counter is in binary
coded decimal (BCD) form. The BCD output needs to be decoded appropriately
before it can be displayed. Decoding a counter has been discussed in the
previous section.

Coming back to the question of counting
pulses, more than one BCD counter stage needs to be used in a cascade
arrangement in order to be able to count up to a larger number of pulses. The
number of BCD counter stages to be used equals the number of decimal digits in
the maximum number of pulses we want to count up to.

With a maximum count of 9999 or 3843, both
would require a four-stage BCD counter arrangement with each stage representing
one decimal digit.

Figure 11.21 shows a cascade arrangement of
four BCD counter stages.

The arrangement works as follows.
Initially, all four counters are in the all 0s state. The counter representing
the decimal digit of 1’s place is clocked by the pulsed signal that needs to be
counted.

The successive flip-flops are clocked by
the MSB of the immediately previous counter stage. The first nine pulses take
1’s place counter to 1001. The tenth pulse resets it to 0000, and ‘1’ to ‘0’
transition at the MSB of 1’s place counter clocks 10’s place counter. 10’s place
counter gets clocked on every tenth input clock pulse.

On the hundredth clock pulse, the MSB of
10’s counter makes a ‘1’ to ‘0’ transition which clocks 100’s place counter.
This counter gets clocked on every successive hundredth input clock pulse. On
the thousandth input clock pulse, the MSB of 100’s counter makes ‘l’ to ‘0’
transition for the first time and clocks 1000’s place counter.

This counter is clocked thereafter on every
successive thousandth input clock pulse. With this background, we can always
tell the output state of the cascade arrangement. For example, immediately
after the 7364th input clock pulse, the state of 1000’s, 100’s, 10’s and 1’s
BCD counters would respectively be 0111, 0011, 0110 and 0100.

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